High temperature low alloy steel

ABSTRACT

A high temperature low alloy steel having a chemical composition consisting essentially of between about (i) 0.05 - 0.25% C, (ii) 0.05 - 0.80% Si, (iii) 0.40 - 1.30% Mn, (iv) 0.30 - 0.20% Mo, (v) 0.002 - 0.015% N in solid solution, (vi) less than 0.010% Sol. Al, (vii) 0 - 0.70% Cr, (viii) 0 - 0.10% V, and (ix) the balance being Fe + incidental impurities. The steel is preferably subjected to a normalizing heat treatment to obtain a high strength steel useful for extended service at temperatures between 400* and 500*C.

United States Patent n 1 Mimino et al.

l l Jan. 2, 1973 54] HIGH TEMPERATURE LOW ALLOY 2,430.15: 8/l949 Malcolm ..15/123 J x STEEL 3,251,682 S/l966 Wade ..75/|Z3 B X 75 Inventors: Tohru Mimillo; Kazuhisa Killoflhitl; $21,; Rebuke "and Kawasaki shi 3,472,707 l0/l969 Phillips .148/36 Kanagawa-ken, Japan [73] Assignee: Nippon Kokan Kahushikl Keisha, P imary Emminer-Hyland Bizot Tokyo, Japan Assistant ExaminerJ. E. Legru Filed: Oct. 26' I970 Attorney-Flynn & Frishauf [2|] Appl. No.: 83,835 [57] ABSTRACT A high temperature low alloy steel having a chemical Foreign Apphcluo" Dita composition consisting essentially of between about Nov. l9, I969 Japan ..44/92413 (i) 025% C, -05 0.80% Si. (iii) 0.40 1.30% Mn, (iv) 0.30 0.20% Mo, (v) 0.002 0.015% 52 us. Cl. ..75/124, 75/126 (3, 75/126 E, N in solid solution, less than 0010% Sol. (vi 75 12 1. 3 3 0 0.70% Cr, (viii) 0 0.10% V, and (ix) the balance [51] Int. Cl. ..C22c 39/04, C22c 39/50 being Fe incidental impurities. The steel is [58] Field of Search ..75/l23, 123 B, I23 1, 123 N, preferably subjected to a normalizing heat treatment 75/124, 126 E, I26 J; l48/36 to obtain a high strength steel useful for extended service at temperatures between 400 and 500C. [56] References Cited UNITED STATES PATENTS 6/1938 Smith et al. -7S/l23 B X 8 Claims, 1 Drawing Figure PATENTED JAN 2 I975 3. 708 280 HIGH TEMPERATURE LOW ALLOY STEEL The present invention relates to an inexpensive low alloy steel having high strength at temperatures within trolled by limiting SolAl to less than 0.010% in order to effect solid solution of N sufficiently. 0.03 0.20% of Mo and 0.01 0.10% of V may be added as additive elements for the purpose of improving creep rupture a range of 450 500C. and has sufficient heat restrength. However, the amount of such expensive elesistance and strength for use in petroleum refining ments as Mo and V is so small that the cost is also low. processes, boilers and general chemical industrial use. In particular, 0.50 l.30% of Mn acts effectively to im- The recent tendency to apply high pressure condiprove high tensile strength, but 0.20 0.70% of Cr is tions in thermal generation units, petroleum refining added to improve both creep rupture strength and high processes and other petroleum industries demands simtemperature tensile strength in the temperature range plified apparatus. High temperature steel applied to of400-500C. such apparatus at temperatures of 450 500C. s Hereinafter, the chemical composition of the present usually a 0.5% Mo steel in contrast to the carbon steel invention steel and that of the conventional steel as a which is used at a temperature of 350 400C. comparative data are shown in the following Table I.

TABLE 1 Examplo l Example 2 Example 3 The conventional steal Carbon 0.15% Mo 0.50% M0 A D C D E F G steel steel steel 0.18 0.13 0.10 0.17 0.16 0.21 0. 0.17 0.11 0.00 0. 2a 0.20 0. is 0.20 0.34 0.24 0.21 0.22 0.07 1.10 0.86 0. 82 1.00 1.15 0. a0 0. 40 0. 40 0. 0.23 0.13 0. a0 0. 42 0. 04 i. 0.00 0.11 0.10 0.12 0. 07 0.07 0.07 i. 0.12 0.03 0. 04 0. 0s 0. 0i 0. 04 i. t. t. 0.000 0.002 0.001 0.000 0.007 0.003 0.003 0. 003 0. 005 0.007 0.007 0.007 0. 00a 0. 007 0.007 0.007 0.007 0.000

Although an 0.15% Mo steel has properties between The alloy steel of the present invention is normalized those of an 0.5% Mo steel and carbon steel, such a steel 6! a t perature of 800 950C. and when the alloy is rarely used for various reasons. steel is used in a boiler tube with a small diameter the In designing apparatus as described above, design 30 cooling speed thereof being relatively fast, it shows a stress at various temperatures is determined as the composition of ferrite and pearlite or a composition lowe t v l among th f ll in values b d up n containing a small amount of bainite in pearlite. There- Japanese Industrial Standards: fore. the hardness is HV I67 l7l, which is relatively a. 0.25 ofthe tensile strength. low, and the cold working property is excellent. Acb. 0.625 ofthe roofstrength, cordingly, annealing is not required after normalizing c. The mean value of stress causing 0.0l percent asls [he case M0 Sleelcreep i hi 1 000 hours Tables 2 and 3 show high temperature tensile d 03 f the minimum va|ue f stress causing rupture strength and creep rupture strength of the alloy steel of within IOO'OOO hours or 06 of the mean va|ue of the present invention. Table 2 shows tensile strength at are 40 room temperature and high temperatures and Table 3 That is to Say (a) and (b) are the values of tens-e shows creep rupture strength at temperatures of 450 0 o I testing at high temperatures for a short period of time and 500 respectively and (c) and (:1) use the smallest value in creep and creep rupture tests as design stress. TABLE 2 Carbon steel is not used for parts which require Room strength at a temperature of over 400 C. because of Temperame 400C 450C 500.cradical decrease in creep rupture strength and high The alloy t /mm it /mm Kg/mm Kg/mm oftlie A 52.5 53.l 48.0 42.6 temperature tensile strength. Although 0.15% Mo steel New" B 55.7 515 525 L6 sees an increase of creep strength to a certain extent at invention C 55.4 3 3,0 47. a temperature of over 400C., high temperature tensile g 22-: 2 -3 1%? 23-3 strength is so low that design stress needs to be F lowered. 0.5% Mo steel has high creep strength at a h b I 2273! 51.2 32.2 3:},

e o l 0 car nstee 4 temperature of over 400 500 C., but has a low tensile mnvmb 045% M0 we] 45x2 4M 3611 293 strength at high temperatures, so that the design stress 55 ional needs to be lowered for practical use. 050% The present invention aims to provide, a low cost, a low alloy steel with high design stress at temperatures TABLE 3 u o r of 450 500 C. by alloying carbon steel with seye ral 6O 450C. Wm 5mm. 0mm kinds of elements in small amounts thereby obtaining crccp rupture crecprupturt: hi h t a d strength strength g emperature tensile strength 11 creep rupture the alloy A 261 g m2 1mm, Strength ol'the a 27.0 10.3 More particularly, the alloy steel of the present in- P C I35 invention D 27.5 18.3 vention consists essentially of 0.05 0.25% C and 0.05 E 303 202 0.80% Si in a killed steel. In this case, 0.002 0.015% G F 28.6 :27 24.3 .l of N dissolves in the alloy steel in the atmosphere. The the carbon eel 3.5 H present invention is characterized in that MN is conconventional L2 5.0

steel 0.50% Mo steel lt will be clear from Table 2 that the alloy steel of the present invention has a much higher tensile strength than the conventional steel at temperatures of 400 to 500C. in every test piece. This results from the following facts: the effect of N, in a state of solid solution in the present low alloy steel, is limited by controlling the amount of Al to be used; the composite effect of N and solid solution and hardening by adding Mn and Cr in Examples l and 2 and the composite effect of Mn, N, solid solution and hardening by adding Mn in Example 3.

It will be clear from Table 3 that creep rupture strength of the alloy steel of the present invention is much higher than that of 0.l5% Mo steel or Carbon steel but slightly lower than that of 0.50% Mo steel. The order of effective elements to be added in order to improve creep rupture strength for a long time is V Mo Cr, but when more than 0.1% of V is added, an extensive improvement of strength is not to be expected. The more the amount of Mo to be added increases, the more creep rupture strength improves, but as shown in the Tables a great improvement of creep rupture strength is not to be expected in case of a large addition. This is shown in the drawing in which the FIGURE is an explanatory chart of creep rupture showing the influence of the amount of Mo addition to the alloy steel of the present invention. The most efficient amount of Mo addition from an economical point of view is within a range of 0.04 0.12%. Cr scarcely contributes to the improvement of creep strength, but Cr addition can be increased because of low cost and in view of its effectiveness to prevent graphitization during the period of use at a high temperature.

As aforementioned, the alloy steel of the present invention has a remarkably high tensile strength at high temperatures and creep rupture strength, so that it is able to improve design stress of high temperature and high pressure apparatus at low cost and can be used in a wide range of fields requiring high strength at high temperatures of up to the extent of 500C. Particularly, the alloy steel of the present invention is suitable for use in boilers, petroleum refining processes and as tube materials for heat exchangers and sheet materials for general chemical industries.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. High temperature low alloy steel suitable for service at temperatures between 400 and 500C having a chemical composition consisting essentially of C 0.05 0.25% Si 0.05 0.80% n 0.40 l.30% Cr 0 0.10% M0 0.03 0.20% N 0.002 0.05%

in solid solution Sol. Al less than 0.0 l0% V 0 a 0. IO'I: Fe incidental impurities balance 2. The steel of claim 1 which after being subjected to a normalizing heat treatment at a temperature of 800 950C retains said nitrogen in solid solution and has a microstructure of (i) ferrite and pearlite, or (ii) ferrite and pearlite containing a small amount of bainite.

3 The high temperature low alloy steel of claim 1 consisting of C 0.05 0.25% Si 0.05 0.80% Mn 0.40 1.30% Cr 0.20 0.70% Mo 0.03 0.20% N 0.002 0.05% in solid solution Sol. Al less than 0.0l0% Fe incidental impurities balance 4. The steel of claim 3 which after being subjected to a normalizing heat treatment at a temperature of 800 950C retains said nitrogen in solid solution and has a microstructure of (i) ferrite and pearlite, or (ii) ferrite and pearlite containing a small amount of bainite.

5. The high temperature low alloy steel of claim I consisting of C 0.05 0.25% Si 0.05 0.80% Mn 0.40 l.30% Cr 0.20 0.70% M0 0.03 0.20% N 0.002 0.015% in solid solution Sol. Al less than 0.0l0% V 0.0l 0. l0% Fe incidental impurities balance 6. The steel of claim 5 which after being subjected to a normalizing heat treatment at a temperature of 800C 950C retains said nitrogen in solid solution and has a microstructure of (i) ferrite and pearlite, or (ii) ferrite and pearlite containing a small amount of bainite.

7. The high temperature low alloy steel of claim 1 consisting of C 0.05 0.25% Si 0.05 0.80% Mn 0.40 l.30% M0 003 0.20% N 0.002 0.0 l 5% in solid solution Sol. Al less than 0.0l0% V 0.0l 0. l0% Fe incidental impurities balance 8. The steel of claim 7 which after being subjected to a normalizing heat treatment at a temperature of 800C 950C retains said nitrogen in solid solution and has a microstructure of (i) ferrite and pearlite, or (ii) ferrite and pearlite containing a small amount of bainite.

I l 1 1K I 

2. The steel of claim 1 which after being subjected to a normalizing heat treatment at a temperature of 800* - 950*C retains said nitrogen in solid solution and has a microstructure of (i) ferrite and pearlite, or (ii) ferrite and pearlite containing a small amount of bainite.
 3. The high temperature low alloy steel of claim 1 consisting of C 0.05 - 0.25% Si 0.05 - 0.80% Mn 0.40 - 1.30% Cr 0.20 - 0.70% Mo 0.03 - 0.20% N 0.002 - 0.015% in solid solution Sol. Al less than 0.010% Fe + incidental impurities balance
 4. The steel of claim 3 which after being subjected to a normalizing heat treatment at a temperature of 800* - 950*C retains said nitrogen in solid solution and has a microstructure of (i) ferrite and pearlite, or (ii) ferrite and pearlite containing a small amount of bainite.
 5. The high temperature low alloy steel of claim 1 consisting of C 0.05 - 0.25% Si 0.05 - 0.80% Mn 0.40 - 1.30% Cr 0.20 - 0.70% Mo 0.03 - 0.20% N 0.002 - 0.015% in solid solution Sol. Al less than 0.010% V 0.01 - 0.10% Fe + incidental impurities balance
 6. The steel of claim 5 which after being subjected to a normalizing heat treatment at a temperature of 800*C - 950*C retains said nitrogen in solid solution and has a microstructure of (i) ferrite and pearlite, or (ii) ferrite and pearlite containing a small amount of bainite.
 7. The high temperature low alloy steel of claim 1 consisting of C 0.05 - 0.25% Si 0.05 - 0.80% Mn 0.40 - 1.30% Mo 0.03 - 0.20% N 0.002 - 0.015% in solid solution Sol. Al less than 0.010%V0.01 -0.10% Fe + incidental impurities balance
 8. The steel of claim 7 which after being subjected to a normalizing heat treatment at a temperature of 800*C - 950*C retains said nitrogen in solid solution and has a microstructure of (i) ferrite and pearlite, or (ii) ferrite and pearlite containing a small amount of bainite. 